Odd endorsements. Ben Carson throws his support to Donald Trump, and Marco Rubio urges Ohio voters to vote for their governor, John Kasich, in next week's primary. Will his desperate effort to deny Trump's Ohio 66 delegates succeed?

Surprising similarities. Bernie Sanders and Donald Trump. They're both railing against trade deals that they say have cost Americans jobs. How did two radically different candidates wind up on the same page?

And Russian hit job? A stunning twist in a deadly mystery. A medical examiner now saying a former top aide to Vladimir Putin died of blunt- force trauma to the head in a luxury Washington hotel room. Why did his family blame his death on a heart attack?

Wolf Blitzer is off today. I'm Brianna Keilar. You're in THE SITUATION ROOM.

We are following the Republican race for the White House. The candidates back on the campaign trail after a surprisingly civil performance in the CNN presidential debate, but that surprise decorum was nowhere in sight at a raucous Donald Trump rally in St. Louis a short time ago. Dozens of protesters were kicked out of the event.

Trump and his rivals all focusing now on next week's primaries and the make-or-break contests for Marco Rubio and John Kasich in their home states, where both men are trailing Trump in the polls.

We're also following the growing mystery surrounding the death of a former top aide to Russian President Vladimir Putin. The man found dead in a luxury Washington hotel of what his family said was a heart attack. And now a shocking twist: the D.C. medical examiner says he died from blunt force trauma to the head. We're covering all of that and more this hour. Our guests including

Republican Congressman Darrell Issa, a member of the House Foreign Affairs Committee, and our correspondents and expert analysts, who are also standing by.

I want to begin now with the Republican race. The candidates now in crunch time with five major primaries just four days away. CNN's Jim Acosta is in Chicago for us.

Jim, Donald Trump is holding a rally there tonight. Tell us about it.

JIM ACOSTA, CNN SENIOR WHITE HOUSE CORRESPONDENT: That's right. Brianna, Donald Trump is looking like a candidate who can't be stopped. He emerged from the CNN debate without a scratch and picked up a big endorsement today from Ben Carson as Trump's rivals are throwing out some creative ideas to slow his momentum before the next big primaries this coming Tuesday.

(BEGIN VIDEOTAPE)

ACOSTA (voice-over): Trying to close one more deal, this time with the Republican Party, Donald Trump is urging his GOP skeptics to embrace his movement.

TRUMP: This doesn't get talked about in the press, the love that's in these rooms. It's love.

ACOSTA: Trump picked up the support of one former rival, Ben Carson, who says he's seeing a new side of the real-estate tycoon.

DR. BEN CARSON (R), FORMER PRESIDENTIAL CANDIDATE: There are two different Donald Trumps. There's the one you see on the stage, and there's the one who's very cerebral, sits there and considers things very carefully.

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: I probably do agree. I think there are two Donald Trumps. I'm a big thinker. I have my ideas, and they're strong and typically they've worked out.

ACOSTA: Defeating just one Trump is enough trouble for Marco Rubio, who took the drastic step of suggesting that his supporters in Ohio actually vote for that state's governor, John Kasich.

SEN. MARCO RUBIO (R-FL), PRESIDENTIAL CANDIDATE: Clearly, John Kasich has a better chance of winning Ohio than I do. And if a voter in Ohio concludes that voting for John Kasich gives us the best chance to stop Donald Trump there, I anticipate that's what they will do.

It's real, real simple. How do you beat Donald Trump? You beat him. ACOSTA: But Rubio is pointing to the CNN debate as proof he's still a

viable contender, especially after he slammed Trump for defending that incendiary comment that Muslims hate the U.S.

TRUMP: I mean a lot of them. I mean a lot of them.

RUBIO: The problem is presidents can't just say anything they want. It has consequences here and around the world.

ACOSTA: Trump seems more concerned about Kasich, unleashing this new attack ad.

UNIDENTIFIED MALE: John Kasich has been an absentee governor, spending most of his time everywhere but Ohio.

ACOSTA: The GOP front-runner again brushed off concerns about the violence flaring up at his rallies, even after one of the supporters sucker-punched a protester.

TRUMP: We've had some violent people as protesters. They're not just people saying, "Oh." These are people that punch. These are people that are violent people.

[17:05:04] ACOSTA: In St. Louis security hauled off more demonstrators, and Trump said he doesn't mind the spectacle.

TRUMP: Can I be honest with you? It adds to the flavor. It really does. Makes it more exciting. I mean, isn't this better than listening to a long, boring speech?

(END VIDEOTAPE)

ACOSTA: And there are more concerns tonight about security here at Trump's rally coming up in Chicago. In just a couple of hours, progressive and civil rights groups here in the city say they will be protesting Trump's super-heated rhetoric on immigration.

Trump's event will be held just a few miles from President Obama's home here in Chicago, Brianna, and I can tell you right now I've been outside this arena where this event -- this event is going to take place later on this evening. There are police circling this entire arena right now, gearing up for what could be a night of trouble here in Chicago, Brianna.

KEILAR: All right. We'll be watching, Jim Acosta in Chicago. Thank you so much.

With Ted Cruz trailing Trump by 100 delegates, the Texas senator is trying to convince voters he's the only one who can take on the billionaire businessman. CNN's Sunlen Serfaty is following the Cruz campaign for us.

So Sunlen, what is the strategy in these final days before Tuesday?

SUNLEN SERFATY, CNN CORRESPONDENT: Well, the strategy, too, Brianna, is all about trying to push this into a two-man race for Ted Cruz. And like a broken record, we will continue to hear him argue that he is the only viable candidate that can take on Donald Trump.

On the campaign trail, he's largely been ignoring his rivals. He did that again today as he campaigned here in Florida. And he went right after Donald Trump, laser-focused on Donald Trump, criticizing him for his debate performance last night, saying Trump offered no solutions to any of the problems that were discussed. And also trying to present Trump as not a real conservative, saying that he's more in line with the Democratic base. Here's what he said earlier today.

(BEGIN VIDEO CLIP)

CRUZ: I believe it's simple. Government is the problem. Donald keeps saying he'll negotiate better deals. He agrees with Barack Obama and Hillary Clinton on everything they do. He just says, "Well, they don't negotiate a good enough deal." He'll do it better.

No, the problem isn't that we need a better deal maker. The problem is we have too much government.

(END VIDEO CLIP)

SERFATY: And also interesting that Ted Cruz today was asked about this idea that Donald Trump said that he does not want to engage in any more primary debates. Ted Cruz was asked and Carly Fiorina, his new surrogate, kind of jumped in his place as she was campaigning with him today; and she said, look, it's because he's scared, basically, and told Donald Trump to man up -- Brianna.

KEILAR: Fighting words. All right, OK. So tell us what Marco Rubio and Ted Cruz are saying about this possibility of a unity ticket?

SERFATY: Yes, this is really fascinating to watch. They both get questions about the notion that they could potentially band up and team up to form this ticket that could potentially be an unstoppable or fierce force against Donald Trump.

Marco Rubio was asked about it here today in Florida. He flat-out denied this is a possibility. He told CNN's Dana Bash that it's something out of "House of Cards," and he's had no conversations to this idea.

And Ted Cruz similarly had a very similar response in recent days when he's asked. He said no. He's not considering it. He's had no conversations. And very similar to Rubio, he says it's kind of like a drama on TV, very clear that right now they are very focused on their campaigns. But of course, we've been talking about very do-or-die stakes for Marco Rubio here in Florida coming up on Tuesday -- Brianna.

KEILAR: Maybe no love lost there. All right, Sunlen, thank you so much.

I want to get more now on all of this with Republican Congressman Darrell Issa of California. He's a member of the House Foreign Affairs and the Judiciary Committee. And he is supporting Marco Rubio, important to note, Congressman. Thank you so much for being with us.

And I know you watched this debate last night. Your candidate had a really strong showing, but is this too late for Marco Rubio? Is this something he needed to do sooner?

REP. DARRELL ISSA (R), CALIFORNIA: Well, the early returns show that, in Florida, Rubio is winning. So the early returns look good, and they've been voting, and it's about 13, 14 percent of the vote that's already in.

But I think talking about last night, I was there in the audience. What I got to see is I got to see Marco being Marco. I saw him calmly going through, using less words than Trump, but in fact, getting the target right on what we should do with Cuba, what we should do with ISIS, what our stance should be in protecting our allies in the Middle East, including and especially Israel.

And because this was so substantive, because they were actually talking issues, what you saw was you saw Donald Trump showing he doesn't understand our relationship with Israel; he certainly doesn't understand the Cuban dynamics. And time and time again, in issue after issue, even Social Security, he kept saying he would simply do it better.

You know, one of the challenges that Marco faces with Donald Trump is he's using Democratic lines. You know, that the problem with government is that it isn't run well enough. Well, this is where Marco Rubio and Cruz and John Kasich, as people who have had to make things work, realize it's just not so.

The fact is, you do not change government boy saying, "Let's work harder." There are good people in government who, in fact, are enforcing too big a government with too many laws. And that's where there's a real difference between Marco Rubio's understanding of what we need to change to move America forward, including fixing Social Security for the next generation, something that last night Donald Trump totally botched.

KEILAR: I want to ask you. You said the early returns are good for Marco Rubio, but when we look at our polling, our latest CNN/ORC poll has Rubio trailing Trump by 14 percent. He recently had this event in Hialeah, which is, you know, a place sort of near to him that I think people are surprised that he didn't get a bigger showing. What does he need to do here in the final days to change voters' minds?

ISSA: Well, first of all, he needs to remind people that when CNN said he was dropping out they were mistaken, and they need to -- you need to drive that message. That's an important part of it. But let's just understand...

KEILAR: We never -- to be clear, Congressman, we did not say that. We said advisers were discussing. So there's a difference.

ISSA: OK. Well, here's the difference. It wasn't true. And it was very clearly said. There was no one willing to come forward and say, "Yes, this was the advisor." But the reality is Marco Rubio has never lost a race in Florida. The

people of Florida have returned him first, you know, obviously to the assembly and then as speaker, now as senator. And on election day, we believe that the early returns, this early voting will be exactly what happens.

And let's understand: "The Washington Post" had it about seven points down. And in all fairness, Donald Trump has never hit that top line number in any of the -- between polling and actual elections, so his 38 is probably 34 or 35.

Is it going to be close? Yes. Did Marco Rubio do something extraordinary, and it was the right thing to do: recognizing that John Kasich is very qualified and, in fact, should be -- should win Ohio. He said to his voters, "Go ahead and support him. He's a good man, and he's not Donald Trump. He understands the issues."

And I'm just sorry that Cruz didn't see it that way and that he's in Florida trying to convince people to vote for Cruz when, in fact, in Florida a vote for Cruz is a vote for Trump.

KEILAR: But I want to ask you about that, because this is pretty extraordinary. A candidate who says, basically, "Don't vote for me. Vote for this other guy who has a better chance of besting this other guy." That happens so rarely. If -- if he isn't thinking of an end game that might include dropping out of this race, why is he doing something like that that a lot of people are saying is a desperate move?

ISSA: Well, it's not a desperate move. It's pragmatic. You have three candidates left.

In a head-on-head with Donald Trump, all three candidates would beat Donald Trump. The problem is, we don't have a head-on-head.

And if we're going through this process of multiple candidates in which you don't have a second round in any of these states and they're not proportional, then in fact, there has to be a certain amount of work a state and win it if you can.

Now, it would be better if Ted Cruz would drop out and realize he's not a uniter; he's a divider. It would be better after Ohio, if John Kasich said, "I don't have a path forward" and dropped out.

ISSA: Well, you know, you can say bested. I didn't say he bested or he didn't. What I said was -- and you can hear it every day -- Ted is a lot like President Obama. He seems to understand it's my way or the highway. There's not a uniter there. I backed -- I called up Marco Rubio and said, "I've worked with you.

I know who you are, and I've listened to you on the campaign trail. I want a uniter. I want someone who would bring our party and our country back together again."

And that's what the voters of Florida are doing, is giving their 99 delegates to Marco Rubio, because he is a uniter.

And when he comes to Cleveland, whether or not he has a clear majority at that point or not, the delegates on the second round, if there's a second round, are going to be looking and realizing that they need to pick the uniter out of the crowd. And that's a process that hasn't happened in a while, but it's clearly possible it could happen here.

KEILAR: All right. Congressman, stay with me. I have many more questions for you. You just said that Ted Cruz should drop out, and I certainly want to follow up on that after a quick break.

(COMMERCIAL BREAK)

[17:18:39] KEILAR: Florida voters will likely decide the future of Marco Rubio's presidential campaign here in next Tuesday's primary. He really has to win his home state for his campaign to remain viable, but polls show that he is trailing Donald Trump by double digits.

We're back now with Rubio supporter, Republican Congressman Darrell Issa of California.

I want to ask you about what I thought was a pretty extraordinary moment last night, where Rubio said that he really regrets his attacks on Donald Trump. He actually said that his kids are embarrassed by it. Do you think that this may have really cost him, even could have cost him the nomination that he abandoned his sunny optimism?

ISSA: Well, certainly, he does regret it, and he said it and has been forthright about it. The interesting thing is Donald Trump can't be embarrassed. He'll say anything about anyone and has.

And that's what's different in this race is you have somebody who regrets appropriately that he got caught up in, if you will, Trumpism, in punching back the way Donald Trump has punched and he's owned up to it and we're moving on. That's the difference of a thinking person who has at least a little bit of humility.

And remember there's nothing that will bring you both power and humility more than being president of the United States. Having great power and recognizing that that responsibility is something that nobody is truly up to. And you've got to be able to listen and digest dozens, sometimes hundreds of different views if you're going to make the right decision on behalf of your country.

This was a practice of George Washington, of Abe Lincoln and others, where they understood that they had the absolute last authority, but they didn't assume that they had all the right answers.

And that's what we're looking at in Florida, is do you want somebody who listens, who brings people together, who's a uniter, or do you want somebody who tells you he knows it all, he's the greatest and he's done the best?

If Donald Trump were the second coming of Jesus Christ, I don't think he could be more self-confident than he is. The fact is, this is a man who doesn't listen to anybody; and that's one of the reasons that many, many people who don't agree on the candidate agree that Donald Trump shouldn't be the nominee.

KEILAR: But if he is the nominee, which there is clearly a better chance that he is the nominee than any of these other Republicans in the field, would you support him?

ISSA: Would you?

KEILAR: You are a Republican, sir. This is your party. Would you support him?

ISSA: The fact -- the fact is, we will go through a process, and we will unify behind somebody who we would like to have beat Hillary.

Look, Hillary Clinton, in my view, should be wearing an ankle bracelet. It's very, very clear that she should not be the next president of the United States.

KEILAR: So then you would support -- you would support Donald Trump over Hillary Clinton?

ISSA: There's no question at all if those are the two names on the ballot, I'll be voting for our nominee.

Would I prefer a uniter? Would I prefer experience? Would I prefer somebody who can heal the wounds that Barack Obama has created in our culture and in our country and in our world? Absolutely. And that's why I sought out and have been working for Marco Rubio. Because on foreign policy, on national defense, he's been doing the kinds of work to prepare him for this.

And, you know, we talked about brokered convention before the break. Remember that William Seward had 37 percent of the delegates going into the convention in 1860, and Abraham Lincoln only had 22 percent. But by the third round, they had picked what they felt was a uniter for their party; and that's the first Republican president. It's not unusual to have elections give you a mixed message delegates find a uniter.

KEILAR: OK, it's -- but if it's not unusual, but I hear you quoting -- you're citing 1860, so obviously, this is somewhat unusual. Do you think -- you've said before that you think this could be a contested convention. Do you think that's more likely?

ISSA: Well, it would be a contested convention. Remember, it hasn't happened much lately, that people talk about 1976 when Reagan contested unsuccessfully or Dewey or others. But it used to happen with some regularity about every 20 years. In this case what's unique that hasn't happened in a while is we

haven't gotten down to two candidates fairly early. If you don't get down to two candidates, you ending up with an odd situation, one in which the person who can solidify 25 or 35 percent of the vote might be the person who cannot get the rest of it.

And in this case, the only person who loses to Hillary Clinton outright in every poll head on head is Donald Trump. Well, Cruz more or less breaks even, and Kasich and particularly Rubio can win.

KEILAR: Congressman, thanks for being with us. We certainly appreciate it.

ISSA: Thank you, Brianna.

KEILAR: And coming up, Dr. Ben Carson endorses Donald Trump, but what did he mean when he said there are two different versions of the billionaire businessman?

Plus more on the rowdy Trump rally that saw dozens of protesters arrested. We have details about security concerns surrounding Trump's rally tonight.

(BEGIN VIDEO CLIP)

TRUMP: If you're a little bit rough, if you're a little rough, do you see the abuse the police were taking back there? They were abused. I mean, the police should be suing them. It shouldn't be the other way around.

(END VIDEO CLIP)

(COMMERCIAL BREAK)

[17:28:31] KEILAR: Donald Trump back on the campaign trail after a notably civil performance in the CNN Republican presidential debate, but a different scene at Trump's rally in St. Louis, where protesters disrupted the rally multiple times. Police say that about 35 people were arrested.

(BEGIN VIDEO CLIP)

TRUMP: Get 'em out. Get 'em outta here. Isn't it great to be at a Trump rally? Really. It's more fun. We'll have to stop for a couple of minutes. We'll have a good time.

(END VIDEO CLIP)

KEILAR: Let's get more now with CNN Politics executive editor Mark Preston. We also have the host of CNN's "SMERCONISH," political commentator Michael Smerconish; CNN senior legal analyst Jeffrey Toobin; and national political reporter for Real Clear Politics, Rebecca Berg.

And Rebecca, this is sort of a new thing that we're seeing here, where Donald Trump is now embracing these incidents. He's -- some of these are even violent incidents, but he's saying this is part of the fun of campaigning.

REBECCA BERG, CNN NATIONAL POLITICAL CORRESPONDENT: He is. Last night he was asked about this in the debate, Brianna, and he said he doesn't condone any sort of violence, of course, but he also didn't really take responsibility for what's happening at his own rallies.

And we should note that this is really the only place where there is violence breaking out, where we're having these protests. Ted Cruz's rallies, Marco Rubio, all these other candidates not having these problems.

Donald Trump would say, well, he has bigger crowds, but his rhetoric also has a very defined "us versus them" quality to it.

[17:30:01] Bad protesters who don't like America versus good Americans who support Donald Trump. And so there is certainly an argument to made -- to be made for him fueling this activity at his rallies.

KEILAR: Jeffrey, as you look at this, what do you make of this?

JEFFREY TOOBIN, CNN SENIOR LEGAL ANALYST: Well, it's going to get worse because it's in the interests of both sides to keep escalating the situation. The protesters really hate Donald Trump and they really think he is a toxic presence. And it's quite clear that Trump is enjoying this process and his supporters love the fact that this is part of his anti-political correctness. You know, don't coddle protesters approach. So it's just heading for something very ugly and nothing looks like it's going to stop it.

KEILAR: That's -- and that's really the question, right, Michael? Does this escalate even beyond what we're seeing? Do you think that this is -- you know, we're just going to see more of the same or do you think this is going to reach a fevered pitch?

MICHAEL SMERCONISH, CNN HOST, "SMERCONISH": Well, it's in nobody's best interest if there's outright violence but I agree with Jeffrey that right now it suits everyone's objective if it stays just at about the boiling point. And you know that line that he loves to utter, if you were to say to me what is Donald Trump's campaign slogan, I'm probably not going to say, "make America great again." At the top of my mind, when I think of Trump now is, "Get 'em out," and it has the intended, I think, meaning of not only whoever is disrupting his event but, of course, all those who are here illegally because he wants them out as well.

KEILAR: All right, Mark, I want to ask you about another development that is really just fascinating today. Marco Rubio, he just seems to have given up on Ohio in order to focus on his home state of Florida where he's trailing Trump in the polls. Here's what he said earlier today.

(BEGIN VIDEO CLIP)

SEN. MARCO RUBIO (R), PRESIDENTIAL CANDIDATE: I suspect that a voter in Ohio that doesn't want Donald Trump to win Ohio may very well conclude that the best way to stop him in Ohio is to vote for John Kasich and I respect that. I'm focused on Florida right now and I'm telling the people in Florida the truth. A vote for Ted Cruz or a vote for John Kasich in Florida is a vote for Donald Trump. The only one who has a chance to beat Donald Trump in Florida is me.

(END VIDEO CLIP)

KEILAR: I mean, when is the last time, Marc, that you saw something like this happen during a presidential election?

MARK PRESTON, CNN POLITICS EXECUTIVE DIRECTOR: I mean, it's just amazing the situation we're in and this time we're in a campaign. If we went back six, seven months ago, Brianna, we wouldn't even be talking about this, we would have never thought we would come to the place where we are right now.

What's really interesting, though, is that Marco Rubio's acknowledgement today that you should vote for John Kasich in Ohio to stop Donald Trump is the second major development in this in many ways because we saw Mitt Romney come out a couple of weeks ago and say anyone but Donald Trump. You now have Marco Rubio, a principal in the race right now, trying to push his supporters in Ohio to go with John Kasich.

In my ways, you've got to wonder what is Marco Rubio's long game right now. Is his long game survival for the presidential race or is his long game to say that he was trying to help protect the Republican Party.

As we're heading into Tuesday night right now, Brianna, the polling shows that Donald Trump has a very good chance of defeating Marco Rubio here right now. There's been a lot of talk about Marco Rubio perhaps running for governor or at least looking at it right now. And this might be part of that play if that's what he's thinking.

KEILAR: And, Rebecca, Mark brings up this idea, is this survival? Which it does make I think many people wonder when they see Marco Rubio saying this, hey, vote for Kasich in Ohio. Some people think it's desperate. I just talked with Darrell Issa who backs Rubio, and he said, no, this is pragmatic. But it sort of gives this feeling of a survivor-esque kind of game show gaming quality to all of this.

BERG: Right. He's just trying to outwit, outplay and outlast all the other candidates, Brianna.

(LAUGHTER)

BERG: As Jeff Probst would say. But, you're right, it's completely strategic and it's not the way that we usually see this game played because usually by this point we're down to two, maybe three candidates who are viable and one clear frontrunner. We do have a clear frontrunner but one that is potentially unacceptable to most or many people within the Republican Party and so we have this very interesting situation on our hands.

It will be interesting to see if voters respond to this call for what we would consider strategic voting as opposed to voting with the person who they actually prefer, especially when there isn't the sort of ad money behind this call to action that you would expect for full saturation.

KEILAR: All right, guys, stay with me. We have much more to discuss and all of this political drama going on. We'll do that after a break.

[17:34:42]

(COMMERCIAL BREAK)

KEILAR: Hillary Clinton and Senator Bernie Sanders face the voters of Ohio in a CNN presidential town hall this Sunday. Then Tuesday's primaries will be our next gauge of how long it could take to settle the race for the Democratic presidential nomination.

CNN's Jeff Zeleny is in Chicago. He is following Bernie Sanders' campaign. And this is a fight, Jeff, even though Hillary Clinton has a pretty hefty delegate lead that could be a long way from being settled.

JEFF ZELENY, CNN SENIOR WASHINGTON CORRESPONDENT: No doubt about it, Brianna. You know as well as anyone the longest this could go is the California primary. That is June 7th. So about three months from now or so.

Now she does have the lead in pledged delegates. Almost mathematically impossible for Bernie Sanders to catch up. But that doesn't mean he's getting out of the race. And even she said she can't blame him.

(BEGIN VIDEOTAPE)

ZELENY (voice-over): Hillary Clinton and Bernie Sanders settling in for the long haul.

[17:40:03] SEN. BERNIE SANDERS (D), PRESIDENTIAL CANDIDATE: On Tuesday, as you know, there are going to be five primaries, including a very big one here in North Carolina. Stand up, fight back, and tell us that our government belongs to all of us.

ZELENY: Sprinting from North Carolina to Ohio to Illinois, Sanders had the campaign trail to himself today, trying to keep the Democratic race alive.

SANDERS: Last Tuesday, everybody had agreed that Hillary Clinton was the sure winner of Michigan. Well, it didn't quite happen that way.

ZELENY: For an afternoon at least, Clinton stepping back into her role as former first lady to celebrate the life of Nancy Reagan. A reminder of Clinton's singular position on the political stage. Sitting between Laura Bush and Roslyn Carter, a one-time senator and secretary of state referred to as Mrs. Bill Clinton in the roster of dignitaries.

She told NBC's Andrea Mitchell that Mrs. Reagan paved the way for her and all first ladies.

HILLARY CLINTON (D), PRESIDENTIAL CANDIDATE: This role is really, you know, so unique. Everyone brings to it up until now her perspective, her experiences. You have to figure out how to support your husband and his agenda, his presidency, but also find a role for yourself.

ZELENY: But this weekend Clinton will be campaigning hard in states voting Tuesday. She's leaving nothing to chance, not even in her native Illinois.

CLINTON: I feel particularly kind of a little emotional. I am a child of Chicago.

ZELENY: She told us she knows this Democratic race has no quick end in sight.

(On camera): Can you understand, though, why Senator Sanders wants to stay in this race until the very end, until California?

CLINTON: Well, I stayed in the race until the very end and, look, I think everyone has to run his or her own campaign, I respect that. If we go to the end, we go to the end, just as I did in '08.

ZELENY (voice-over): As their Democratic fight goes on, she and Sanders are increasingly fixating on another rival, Donald Trump.

SANDERS: The American people in my view will not support a president who insults Mexicans, who insults Muslims.

ZELENY: He's becoming a rallying cry to fire up the Democratic faithful.

CLINTON: I've been getting messages from a lot of the leaders that I know from around the world. And their message basically is what is happening?

ZELENY: In Florida today, Trump had words of his own about Democratic voters.

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: We're going to have Democrats for Trump. But it's going to be much bigger. We're going to get tremendous numbers of Democrats.

ZELENY: If Trump is the Republican nominee, Clinton says his lore may not be as impressive as he thinks.

CLINTON: Look, as I've said, I've gotten more votes than he has. I think he has -- if you really analyzed it -- a pretty narrow base. But look, we'll find out. If he gets nominated, we're going to have a very vigorous general election if I'm the nominee.

ZELENY (on camera): Is he still yet to be defined, though? I mean, this has been a primary campaign.

CLINTON: We'll wait and find out.

(END VIDEOTAPE)

ZELENY: Now an unusual retraction today from Secretary Clinton herself. While she was in Simi Valley attending the funeral for Mrs. Reagan, she praised the Reagans for leading a national conversation on AIDS. She was quickly criticized for that so she released this statement a short time ago, it reads like this. "While the Reagans were strong advocates for stem cell research and finding a cure for Alzheimer's, I misspoke about their record on HIV and AIDS. For that, I'm sorry."

Brianna, an unusual apology from the secretary so quickly after she praised them. This of course is a big issue in the gay community for how the Reagans reacted to the AIDS crisis in the 1980s.

KEILAR: Yes. A rare apology and also that was a really rare poolside interview that you snagged with her. That was great to see.

Jeff Zeleny, for us in Chicago. Thanks so much.

I want to bring back our political experts to talk about the Democratic race.

And, Mark, I know that you're learning some changes really to the messaging that we're going to be hearing out Democrats over the next few days. What's going on there?

PRESTON: Yes. Well, Brianna, first of all, we're here in Miami, Florida, right now and the candidates are not here. Florida right now is in the rearview mirror as the candidates had moved up, certainly Bernie Sanders, up to North Carolina but it's really moving into the industrial Midwest. Hillary Clinton will join him there tomorrow.

The fight is right now on these blue-collar voters. Now Bernie Sanders' campaign has been saying over and over again this is where they start to gain momentum. We saw that happen in Michigan with the surprise win. Now they have to do well in Missouri, Illinois, and of course Ohio. Missouri might be their best chance of winning right now.

Hillary Clinton, though, expect to hear this messaging over the next couple of days. Net delegates. Even if Bernie Sanders is able to win two or three of these states, will he receive more delegates than Hillary Clinton? That is going to be the messaging or part of the messaging out of the Clinton campaign. Also expect her to talk about the fact that her economic policies are more realistic.

Bernie Sanders, though, will say that he's a fighter for the middle class. Expect to hear a lot about trade and of course CNN's town hall on Sunday night. This will be in full force and certainly as they make their pitch to the voters heading into Tuesday, Brianna.

[17:45:11] KEILAR: Yes. And that's a town hall, Michael, that we're going to see in Ohio. I think some people are looking at Bernie Sanders' upset in Michigan and wondering is he going to do this again in Ohio? Do you think it's really a possibility? SMERCONISH: Well, size matters. He needs to win big states. More

than half the delegates are going to be allocated from just six states and two of them are, to Mark's point, Illinois and Ohio. And the issue is, where can he replicate Michigan? Where is there a state that maybe he's down 20 in the polls that he can pull off with a lunch pail crowd, primarily white voters, and the Sanders campaign has highlighted those two states and we will know Tuesday night.

KEILAR: Well, Rebecca, Bernie's chances in Ohio, how do you see them, especially when the polls in Michigan were so different than the outcome?

BERG: I see his chances as very good there, Brianna, because when I look at what he did in Michigan, especially on the issue of trade, so hitting Hillary Clinton really hard on her past support for free trade agreements, helping to craft TPA and kind of waffling on the Transpacific Partnership, that really worked for Bernie Sanders in Michigan with these white working class voters who fear that their jobs are being shipped overseas. And so if he's able to continue hitting her on that message in Ohio, I think it could be very successful.

KEILAR: Final word to you, Jeffrey. With Sanders talking about staying in the race here, what is he hoping to get out of this, if he cannot catch up to Clinton in the delegate count?

TOOBIN: I mean, let's give the guy a chance, he just won Michigan. I mean, he's doing fine.

BERG: He did.

TOOBIN: And Hillary Clinton's big slogan is net delegates? Boy, that's almost as pathetic as Marco Rubio saying vote for the other guy. That's how -- I mean, you know, can I just say one thing, Brianna? People may be a little confused about why Hillary Clinton apologized. The reason she apologized is that Ronald Reagan let tens of thousands of people die of AIDS and HIV before any -- before he uttered a word about the crisis, so to praise him was pretty outrageous.

She was right to apologize, even in the context of a funeral where people are praised rather than, you know, honestly appraised.

KEILAR: Yes. I think that's a really good point that you make there.

All right, Jeffrey Toobin, Michael Smerconish, Mark Preston and Rebecca Berg, thank you so much to all of you.

And I do want to give you a quick reminder. Be sure to watch the CNN Ohio Democratic presidential town hall with TV1 this Sunday at 8:00 p.m. Eastern. I'll be there. I'm sure it's going to be a great one.

And coming up, a one-time ally of Vladimir Putin found dead at an upscale hotel right here in Washington, D.C. Was this a heart attack or a hit job?

Also ahead, North Korea searches for a missing submarine while the Pentagon worries about what its crew may have been up to.

(COMMERCIAL BREAK)

[17:52:19] KEILAR: We're learning some shocking new details about a mysterious death at a hotel here in Washington. It turns out that a one-time aide to Russian President Vladimir Putin did not die of a heart attack.

Let's get the latest now from CNN's Brian Todd who's at the hotel -- Brian.

BRIAN TODD, CNN CORRESPONDENT: Brianna, this is indeed right out of a Cold War spy novel. As you mentioned, the family of this former aide to Vladimir Putin had said that he had died at this hotel of a heart attack. But new information tonight from the D.C. medical examiner's office really elevates this mystery and raises many more questions.

(BEGIN VIDEOTAPE)

TODD (voice-over): A power broker in Moscow connected to Vladimir Putin found dead in a Washington hotel room. When Mikhail Lesin's body was discovered in November, his family told Russian media he died of a heart attack. Tonight the D.C. medical examiner's office says Lesin died from blunt force injuries to the head, neck, torso, and extremities.

MITCH CREDIE, FORMER D.C. HOMICIDE DETECTIVE: More than likely just based on the injuries he was involved with something physical with someone and what I would do as the detective on this case is to first determine where it occurred.

TODD: According to some media reports, there were no signs of forced entry or foul play in Lesin's room at the Dupont Circle Hotel and there are questions about whether he was injured before he entered the hotel. The D.C. police, the lead instigative agency, did not respond to CNN's multiple requests for details.

We pressed the hotel for information on witnesses and surveillance footage. They wouldn't give it.

Lesin's background raises suspicions. He was a longtime adviser to Vladimir Putin, one-time head of the Russian government's media arm. A U.S. senator called Lesin one of the Kremlin's top sensors. He later headed Gazprom Media which controls several popular Russian TV stations.

In 2014, Senator Roger Wicker asked the Justice Department to investigate Lesin for money laundering and corruption. Wicker claimed Lesin had acquired $28 million worth of property in California. Lesin denied it. The Justice Department referred the case to its criminal division and the FBI, but it's not clear if there was ever an investigation and neither agency would comment to CNN.

From opposition leader Boris Nemtsov to former KGB spy Alexander Litvinenko, does Mikhail Lesin joined the list of people who could have damaged Putin and wound up dead? A former D.C. homicide detective says there are still too many

unanswered questions.

CREDIE: If there's no forced entry into the room, there are no signs of foul play, I want to be able to determine if it occurred there. He could have been assaulted outside the hotel. Came to the hotel. Passed out and died. But if that's the case, someone had to have seen him enter the hotel.

(END VIDEOTAPE)

TODD: A Russian official who we spoke with would not comment on whether Mikhail Lesin might have been murdered. Would not comment on the money laundering accusations. When we asked about any possible connections between President Putin and Lesin's death, this official said those kinds of speculations are inappropriate -- Brianna.

[17:55:12] KEILAR: All right. Brian Todd for us on that very mysterious story.

And I do want to say we're standing by right now for a Donald Trump rally in Chicago. Will there be more trouble with protesters just like we saw at today's earlier rally in St. Louis? And also the latest on the search for a missing North Korean submarine and whether its mission could pose a threat to the U.S.

(COMMERCIAL BREAK)

KEILAR: Happening now, civility or civil war? Republicans go from the calm of the CNN debate to the chaos of a Donald Trump rally. Interrupted by protesters, Trump goes on the attack.